1. Field of the Invention:
The invention relates to a process for producing benzyl alcohols from the corresponding aromatic carboxylic acids.
Among benzyl alcohols, there are many compounds useful as intermediates for agricultural chemicals and pharmaceuticals or as perfumes. However, it has not been successful to date to produce them at low costs on an industrial scale.
2. Prior Art of the Invention
It is well known to reduce aromatic carboxylic acids electrolytically in aqueous acidic solutions. For instance, it has been proposed to submit aromatic carboxylic acids to electroreduction at 70.degree. C. in the presence of sodium amalgam and 15 wt. % sulfuric acid [Bericht 38, 1752 (1905)]. However, this process gives so low yield that it has not been adopted industrially.
The present inventors have previously proposed to reduce m-hydroxybenzoic acid or esters thereof electrolytically at a pH of 4 or less in an aqueous solution or in a water-soluble organic solvent (Japanese Patent Laid-Open No. 234987/1985).
The inventors have also proposed to use a cation-exchange membrane as a diaphragm and add a quaternary ammonium salt as a supporting electrolyte in the electroreduction (Japanese Patent Laid-Open No. 243293/1985).
The inventors have further proposed an electroreduction process for producing high purity p-xylylene glycol from other benzoic acids than m-hydroxybenzoic acid, for example, from terephthalic acid (Japanese Patent Laid-Open No. 297482/1987).
In addition, the inventors have proposed an electroreduction process of m-phenoxybenzoic acid as a benzoic acid other than m-hydroxybenzoic acid (Japanese Patent Laid-Open Nos. 17188/1988 and 192883/1988).
The electroreduction of these aromatic carboxylic acids are usually carried out in an aqueous acidic solution. No particular restrictions are imposed on the aqueous acidic solution so far as the acidic substance contained therein is inert to the electrolytic reaction at the cathode. Costwise, however, it is generally desirable to use mineral acids, and hence sulfuric acid has been particularly used from the standpoint of material and yield.
Cathode materials used in the electrolytic reaction include those involving high hydrogen overvoltages, specifically, zinc, lead, cadmium, mercury, and the like.
In the opposing anode, it is common to use metallic materials that are not corroded in aqueous sulfuric acid solutions or otherwise do not affect the cathodic reaction adversely even if they are taken into solution as metallic ions. Platinum, lead oxide, lead and lead alloys are being generally used.
However, the foregoing anode materials including platinum, lead oxide, lead and lead alloys have high oxygen overvoltages at usual supply voltages (0.1 A/dm.sup.2 -100 A/dm.sup.2) and therefore their oxidizing abilities are high. Consequently, when the electrolytic reaction of aromatic carboxylic acids is carried out in a single electrolytic cell in which the anode and cathode compartment are not separated from each other, oxidation as well as reduction take place simultaneously, resulting in a significant decrease in the yield of the intended benzyl alcohols.
In order that the oxidation reaction on the surface of the anode is suppressed and the intended product is obtained in a high yield, it has been necessary to conduct the electrolytic reaction in an anode-cathode separated electrolytic cell in which the anode compartment is separated from the cathode compartment by a diaphragm like an ion-exchange membrane, and feed the raw material only to the cathode compartment.
However, use of the anode-cathode separated electrolytic cell in the industrialization of the electrolytic reaction unavoidably increases the power cost due to the resistance of the diaphragm, increases the cost of expendables as the diaphragm, and causes mass transfer from the cathode compartment to the anode compartment during the reaction. The transfer loss leads to a reduction in the yield, while the two solution tanks required for the anode and cathode renders the apparatus complicated. Further, the installation of the diaphragm has imposed a restriction on the shapes of the electrolytic cell and the electrodes.